Transmission control circuits



Oct. 4, 1932. B. ca. BJORNSON ETAL 1,880,715

TRANSMISSION CONTROL "cincuiws Filed Sept. 10. 1950 2 Sheets-Sheet 2 8. G. BJORNSON INVENTORS AWHORRDI, JR.

A.C.NORWN' By J ATTORNEY Patented Oct. 4, 1932 FIQE BJoRN e. BJoR-NsoN, on NEW YORK, 1v. Y., AND ARTHUR W.HORTO1\T, an; OF EAST onANGE,

AND ANDREWO. NORWINE, OF IEASSAIC, NEW JERSEY,ASS IGNORS TO BELL TELE- PHONE LABORATORIES, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK TRANSMISSION. CONTROL CIRCUITS Application filed September 10, 1930. Serial no. se ate.

' The invention relates to two-way signaling systems and particularlyto the signal controlled means employed in such systems for conditioning the circuits thereof for various purposes such as to suppress disturbances, i. e., echoes, and to prevent singing.

An object of the invention is to improve the operation characteristics of such signal controlled means in two-way signaling systems.

The invention is particularly directed to the signal-controlled means utilized in con nection with two-way signaling systems for suppressing echoes, such as echo suppressors, although it is applicable as well to signal controlled means utilized in connection with twoway signaling systems for other purposes, for example, so-called reaction suppressors, antisinging devices and voice operated repeaters.

An echo suppressor is that type of signal controlled device well-known in the art utilized in connection with two-way signal ng systems for effectively disabling one of the associated one-way transmission circuits thereof while signal transmission is taking place over another oppositely directed one way circuit of the system, thereby preventing echo currents or reflected currents from being transmitted back to the transmitting end of the system and causing a disturbance or singing. The disabling apparatus may comprise, for example, a short circuit which may be closed across one of the one-way circuits by the operation of a mechanical relay in the output of a vacuum tube amplifier-rectifier having its input connected'across the one-way circuit for transmission in the opposite direction.

This invention is concerned mainly with improvements in the type of signal controlled circuits employed in connection with two way signaling systems for the above mentioned purposes, in which they sensitivity is controlled in accordance withthe amotmt of line noise in the system, so as to prevent false operation thereof by line noise, while maintaining at all times the circuits in th most sensitive condition as regards operation by signals. This type of signal controlled circuit when employed for suppressing echoes is known as a variable sensitivity echo sup sor is operated by signals from the other oneway path, the inputs to both the suppressor and the sensitivity reduction circuit are simultaneously disabled, and the first suppres sor is maintained at maximum sensitivity until signal transmission ceases. The suppressors may then be locked up from noise until the sensitivity control circuit has had time to function.

In th evariable sensitivity signal controlled system of the invention, the above men'- tioned disadvantages are eliminated by a construction enabling thesensitivity of the signal controlled suppression devices to be determined'b'y the amount of noise on the one-way transmission path on which the noise is a maximum, and in which the sensitivities of both signal controlled suppression devices associatedwith the two oppositely di rected transmission paths are reduced simultaneously and by the same amount. A preferred embodiment of the invention is applicableto echo suppressors or anti-singing devices of the type in which mechanical relays are utilized for disabling the one-way transmissionpaths. The master relays controlling the disabling of the one-way paths are normally biased so as to operate in their most sensitive condition. Themaster relays are provided with. additional windings which, when energized, will reduce thesensitivity of the relays. These additional windings are energized in accordance with the amount of line noise on the, one-way transmission path in which noise is a maimum through the consecutive operation of a chain of relays arranged to respond to line noise but not to signals.

The nature of the invention, its purpose and advantages will be clear from the following detailed description thereof when read in connection with the accompanying drawings, Figs. 1 and 2 of which show diagrammatically four-wire telephone repeating systems employing echo suppressors embodying different modifications of the invention.

The four-wire telephone repeating system of Fig. 1 comprises the one-way transmission path EA including the one-way amplifying devices 1 and 2 forrepeating telephonic signals in the direction from west to east between a west two-way line section L and an east two-way line section L and the oneway transmission path VA including. the one-way amplifying devices 3 and 4 for repeating telephonic signals in the direction from east to west between the east line section L and the west line section L The input of theeast path EA and the output of the west path WA are associated in energy transmitting relation with the line section L andin conjugate relation with each other, and the input of thewestpath WA and the output of the east path E are connected in energy transmitting relation with the line section L and in conjugate relation with each other, by means of the hybrid coil transformer H and H and associated balancing networks N and N respectively, in the manner well known in the art.

Connected across the transmission path EA between the output of amplifier 1 and the input of amplifier 2 therein is a control circuit 5 including a wave-controlled, relaycontrolling device 6, and the operating winding of the master echo suppressor relay M and the operating winding of an auxiliary relay A connected in series across the output 7 of the device 6. Connected across the transmission path WA between the output of the amplifier 3 and the input of the amplifier 4 is a control circuit 9 comprising the wavecontrolled, relay-controlling device 10, and

j the operating winding 12 of the master echo suppressor relay M and the operating winding 12 of an auxiliary relayA connected in series across the output of the device 10.

Adjacent the transmission path WA is a relay S which, when energized, is adapted to close the normally open contacts 13 in the connection 14 so as to short-circuit the outgoing portion of the path WVA at the points 15. Relay S is arranged to be supplied with energizing current from the battery 16 over the conductor 17 when the'normally open contacts 18 therein are closed by operation of the holdover relay H0 Adjacent the transmission path EA is the'relay S which,

' when energized, is adapted to operate to close the normally open contacts 19 in the connection 20 so as to short-circuit the outgoing portion of the path EA at the points 21. The relay S is arranged to be supplied with energizing current from the battery'22 over the conductor 23 when the normally open contacts 24 therein are closed by operation of the holdover relay H0 The holdover relay HO has three windings, a biasing-winding 25, an operating winding 26 and a holdover winding 27. The holdover relay HO has three windings, a biasing winding 28, an operating winding 29 and a holdover winding 30. The biasing winding 25 of holdover relay H0 and the biasing winding 28 of holdover relay H0 are connected in series and are maintained energized at all times by current from the battery 31 through resistance 32. The operating winding 26 of holdover relay H0 is normally short-circuited through the normally closed contacts 31 of themaster echo suppressor relay M Similarly, the operating winding 29 of holdover relay H0 is normally short-circuited through the normally closed contacts 32 of the master echo suppressor relay M When the master relay M operates to open its normally closed contacts 31 the short circuit around the 0 eratin winding 26 of holdover relay H 1 is removed and the winding is energized by current from the battery 33, over a path extending from ground through battery 33, resistance 34, winding 26 of relay H0 conductor 35 and the normally closed contacts 36 of the-holdover relay H0 to ground. Similarly, when the master echo suppressor relay M operates to open its normally closed contacts 32, .the short circuit around the operating winding 29 of relay H0 is removed and that winding is energized by current from the battery 37 over a path extending from ground through battery 37, resistance 38, winding 29, conductor 39 and the normally closed contacts 40 of relay H0 to ground. The holdover winding 27 of holdover relay HO is normally deenergized. When'master relay M operates to close its contacts 41, the winding 27 is energized by current from the battery 42 over. a path extending from ground through battery 42, resistance 43, winding 27, resistance 44, contacts 41, conductor 35, and the normally closed contacts 36 of holdover relay H0 to ground. Similarly, the holdover winding 30 of hold over relay H0 is normally deenergized. When the master relay M operates to close its contacts 45, winding 30 is energized by mm rent from the battery 46 over a path extending from ground through battery 46, resistance 47, .winding 30, resistance 48, contacts 45, conductor 39 and the normally closed contacts 40 of holdover relay H0 to ground.

While the holdover winding 27 of relay H0 is being energized, the condenser 49 is tance 47 also charged up by current from the battery 42. lVhen relay M releasesto break the energizing circuit for winding 27 by opening contacts 41, condenser 49 will discharge slowly through winding 27 and resistance as so as to maintain winding 27 energized for an additional interval of time. Therefore the relay H will remain operated to hold its contacts 18 closed and thus the short-circuit ing relay S energized by current from battery 16 for this additional interval of time after the relay M has rel ased. Similarly while the holdover winding 30 of relay HO is being energized the condenser 50 is also being charged up by currentfrom battery 46. When relay M releases to break the energizing circuitfor winding 30 by opening contacts 45, the condenser 50 will discharge slowly through winding 30 and resis to maintain winding 30 energized for an additional interval of time. Therefore, relay H0 will remain operated to hold its contacts 24 closed, and thus the shortcircuiting relay S energized by current from battery 22 for this additional interval of time after relay TV 2 has released.

The resistance 43 and the condenser 1-9 associated with holdover relay H0 are given suitable values so that the additional length of time for which relay H0 will be main tained operated after transmission of the West to east speech currents to the control circuit 5 has ceased, is sufficient to insure that the short-circuiting connection across path TVA at the points is maintained long enough to block therein any echo or reflected currents due to the preceding west to east speech currents; in other words, the holdover action of holdover relay H0 is made such that the line WA is maintained shorted at the points 15 for aperiod of time at least equal to the transmission time over the path EA between the point of connection of the circuit 5 to the path EA through the hybrid coil H plus the transmission time from the hybrid coil H over the path WA to the points 15 therein. Similarly, the resistance 47 and condenser 50 associated with the holdover winding of relay HO are given such values that the additional length of time for which the relay H0 will be maintained operated after the transmission of east to west speech currents over the path TVA to control circuit 9 has ceased is sutlicient to insure that the short-circuiting connection across the path EA at the points 21 is maintained long enough to-block any echoes or reflected current due to the preceding east to west speech current. This latter interval of time will be substantially equal to the transmission time over the path WA between the point of connection of the control circuit 9 to the path TVA through the hybrid coil H plus the transmission time over path EA. from hybrid coil H to the blocking points 21 in the path EA. It may be desirable to make the holdover periods of relays H0 and H0 slightly longer than the intervals described to prevent the short-circuiting connections across the paths WA and EA respectively from being removed at the trailing end of a word or syllable.

The master echo suppressor relay 1 1 has a biasing winding 51 which is maintained energized at all times by current from the battery 52 through the resistance 52 so that the relay would operate in the least sensitive condition were it not for winding 56. Similarly, the master relay M has a biasing winding 54 which is maintained energized at all times by current from the battery 55 through the resistance 55 so that the master relay M would operate in the least sensitive condition, were it not for winding 57. The master relay M has a third winding56 so wound as to oppose the action of the operating winding 7 thereof. Similarly, master relay M has a third winding 57 so wound as to 0ppose the action of the operating winding 11 thereof. The third windings 56 and 57 of relays M and N respectively are utilized to apply back bias to these relays opposing the normal bias produced by the biasing windings 51 and 54, respectively. thereof. The winding 56 of master relay M and the winding 57 of master relay M are connected in series so as to be normally supplied with energizing current from the battery 58 through the resistance 59 and the normally closed contacts 60 of the auxiliary relay A The elements in this series circuit are so chosen that the. amount of back bias normally applied is the same for each master relay, and is S'LllfiClGIlt to allow the relays M and M to operate in their most sensitivecondition. This normally applied, back bias is removed in one step when auxiliary relay A releases to open its contacts 60 causing the sensitivity of each master relayhl and M to be reduced by an equal amount to the point where these relays will just not be operated by the noise received by them from the paths EA and WA, respectively, although responsive to the speech waves received from these paths.

The auxiliary relay A has two windings, a biasing winding 61 and an operating winding 62. The biasing winding 61 is at all times energized by current from battery 63 through the resistance 64. The operating winding 62 of relay A is normally energized by current from the battery 65 over a path extending from ground through battery 65, resistance 66, winding 62 and the normally closed contacts 69 of the auxiliary relay A to ground, so as to hold the relay contacts 60 closed.

A. condenser 67 and a resistance 68 are connected in series across the winding 62 and the'resistance 66, so that condenser 67 will be normally charged by current from battery through resistance 68. When the normally closed contacts 69 of the auxiliary relay A are opened by operation of that relay, the normal energizing circuit for the operating winding 62 of relay A is broken, but the relay A will not release for a definite time thereafter because the winding 62 will be maintained energized while the condenser 67 discharges through that winding and the resistances 66 and 68. This definite period of time may be made of any desired length by proper choice of the values of condenser 67, resistances 66 and 68 to regulate the time constants of the relay A The time constants of relay A are made such that the relay will not release unless the contacts 69 of auxiliary relay A remain open for a time longer than the duration of the average speech syllable plus the holdover period for relay A Thus, the contacts 69 will never be closed long enough by operation of relay 23. when speech energy alone is supplied to the preceding relays (A and A in the chain to cause relay A to release. The speech waves applied to relay A or A therefore,

will not be efi'ective to cause any reduction in the sensitivity of the master suppressor relays M and M through relays A and A Auxiliary relay A which is controlled by operation of either auxiliary relay A or A is provided to bridge the periods between peaks ofnoise in order to release auxiliary relav A on intermittent noise. It controls relay A through its contacts 69 when auxiliary relay A or A operates frequently enough so that the holdover of auxiliary relay A, allows auxiliary relay A to-release.

Auxiliary relay'A has two windings, a biasing winding 70 and an operating winding 71. The biasing winding 70 is always energized by current from battery 72 tliroug h resistance 73. The operating winding 71 is arranged to be energized by current from the battery 7% through resistance 75 when either the normally open contacts 76 ofauxiliary relay A or the normally open contacts 7 7 of relay A are closed by operation or" the respective relays.

A resistance 78 and a condenser 79 are connected in series across the winding 71 of relay A and the resistance 75. l/Vhen the contacts 7 6 of relay A or contacts 77 of relay A are closed, the current from battery 74:, besides energizing winding 71, also'charges the condenser 79. When the energizing circuit for winding 71 on relay A is broken by the opening of contacts 76 of relay A or contacts 77 of relay A the condenser 79 slowly discharges through the resistances 7 8 and 75 and the winding 71, causing the winding 71 to be maintained energized, and thus the relay A to be maintained operated to hold its contacts 69 open, for a definite holdover period of time after the supply of energizing current towinding 71 from battery 7 l ceases.

The value of the condenser 79 and of the resistances 78 and 75 are chosen so as to give the relay A a holdover in its operation suflicient to bridge the interval between peaks of intermittent noise of a particular type which will be referred to below.

The auxiliary relay A has, in addition to its operating winding 8, a biasing winding 80 which is always maintained energized by cur rent from the battery 81 through the resistance 82. The value of resistance 82 is preferably so chosen that the relay A is about 3 decibels more sensitive than the master echo suppressor relay 1 Similarly, the auxiliary relay A has in addition to its operating winding 12 a biasing winding 83 which is always maintained energized by current from the battery 84 through the resistance 85. The value of the resistance 85 is also preferably so chosen that the relay A is about 3 decibels more sensitive than the master relay M As the two relays A and A have equal sensitivities, the one supplied with currents of the greater amplitudes will operate first.

A condenser 86 of suitable value is connected across the series circuit comprising the winding 7 of relay M and the winding 8 of relay A to prevent chattering of the contacts of these relays. Similarly, a condenser 87 of suitable value is connected across the winding 11 of relay M and the winding 12 of relay A to prevent chattering of the contacts of these relays. r The wave-controlled, relay-controlling devices 6 and 10 may be vacuum tube, amplifier-rectifier, circuits of the type well known in the art or any other devices which will respond to alternating current impressed upon their inputs to operate mechanical relays in their outputs.

The operation of the system of Fig. 1 as a whole will now be described.

As the operation of the system when speech waves are being transmitted in the direction from west to east is similar to the operation when speech waves are being transmitted in the direction from east to west, only the former case need be described. It will be assumed that speech waves are being transmitted from west to east over the transmission path EA between lines sections L and L and that at that time no speech waves are being transmitted in the direction from east to west over the transmission path WA. It will also be assumed that there is noise on both the path and'the path NA and that the amount of noise on the path EA is greater than that on the path WA at this time.

The speech currents transmitted over the path EA will be amplified by the amplifier 1 therein and the amplified waves divided between the input of the amplifier 2 in the path EA and the input of control circuit 5. The main portion of the speech waves in the output of amplifier l in the path EA will be amplified by the amplifier 2, and the amplified waves will be impressed by the hybrid coil H upon the line section L and transmitted thereover to the listening subscriber.

A portion of the amplified speech waves in the output of the amplifier 1 in the path EA together with a portion of the noise Waves which are present in the output of amplifier 1 will be impressed upon the input of the wave-controlled, relay-controlling device 6 in control circuit causing that device to be operated to energize the operating winding 7 of relay M and the operating winding 8 of auxiliary relay A The master relays M and M and the auxiliary relays A and A are designed to operate at a syllabic frequency. The frequency of their operation depends upon the sensitivity of the associated ampli- Her-rectifier devices, the type of the impressed waves and their rapidity and various other factors including the relay design. Auxiliary relay A being more sensitive than relay M will operate in advance of M to start the chain of operations which will be described later.

Relay M will then operate to close its normallyopen contacts 41 and to open its normally closed contacts 31. The opening of contacts 31 will cause the short circuit around the operating winding-26 of relay H0 to be removed, allowing that winding to be energized by current from battery 33. The closing of contacts 41 causes the holdover Winding 27 of relay H0 also to be energized by current from battery 42.

Relay H0 will operate to close its contacts 18 in conductor 17, and to open its contacts in the conductor 39. The opening of contacts 40 in conductor 39 will efiectively disable the energizing circuit for winding 29 of relay H0 thus preventing that relay from being thereafter operated to cause the short-circuiting of path EA by operation of the relay S due to later false operation of relay The closing of contacts 18 in conductor 17 will cause the windings of relay S to be energized by current from battery 16. Relay S will then operate to close the normally opened contacts 13 in the connection 14 causing the outgoing portion of the transmission path WA to be short-'circuited thereby at the points 15. The path WA will be maintained shortcircuited as long as speech waves are being received by the relay M from the transmission path EA, and due to the holdover action of the relay vH0 will be maintained shortcircuited for a definite interval after that time, while condenser 49 which was charged by current from battery 42 through resistance 43 and winding 27 during the time of energization of the holdover Winding 27 of relay H0 is discharging through the resistance 43. As stated above, this holdover period is made long enough to insure that the path WA is blocked long enough to prevent all echoes or reflected currents in the path WA 7 to close and maintain closed its contacts 76.

When contacts 76 are closed the operating winding 71 of relay A is energized by current from battery 74 over a path extending from ground through battery 74., resistance 75, winding 71 and contacts 76 of relay A to ground. Relay A will then operate to open its contacts 69 to break the energizing circ-iiit for the operating winding 62 of relay By proper selection of the values of the condenser 79 and the resistances 78 and 75 comprising the holdover circuit of relay A and the values of condenser 67 and resistances 68 and 66 comprising the holdover circuit for relay A the holdover of these two relays are made such that relay A will not release for operation of relay A by the syllabic variations in speech but will release for operation of relay A on intermittent noise of a certain type. 1

The'A relay is providedto introduce a holdover act-ion to take care of certain types of noise especially those with pronounced low frequency components which would cause auxiliary relay A or auxiliary A to Vibrate freely without effecting operation of the auxiliary relay A The elements in the holdover circuit are preferably such "that aux- T iliary relay A. gives a holdover-action somewhat less than that of relay HO or relay H0 and yet sufliciently long to bridge the intervals between peaks of noise of the above mentioned type. The holdover time of auxiliary relay A preferably should be greater than .01 seconds to bridge the intervals between cycle noise peaks. A holdover time for relay A of between .05 and .10 seconds was found to give the best results with most noises. To prevent undue operation by speech the release time of auxiliary relay A preferably should be greater than .75 seconds.

' The noise waves impressed with the speech waves from the output of the device 6 upon the operating windings of auxiliary relay A are steady compared with the fluctuating speech waves. They will cause the contacts 76 of that relay to be maintained closed long enough to cause such operation of auxiliary relay A. that its contacts 69 will remain open for a suflicient length of time to enable relay 3 to release to open its contacts 60., The opening of the contacts 60 will break the energizing circuit for winding 56 of master relay ated by .the noise impressed on their operating windings, While still being responsive to the impressed speech waves. Increased noise on a telephone line will generally permit operation of the suppressor relays at reduced sensitivity due to the fact that the talkers tend to raise their voices to get through the noise and due to the masking eiiect of the noise on the weak echoes.

. A part of the noise which is present in the output of amplifier 3 in the transmission path WA at the time speech waves are being transmitted over the path EA, and which according to our assumption is of less magnitude than the noise on path EA, will be diverted into the control circuit 9 causing the operation of the wave-controlled, relay-controlled device 10,to energize the operating windings 12 and 11 of the auxiliary relay A and the master echo suppressor relay M respectively. The energization of the winding 11 of master relay M by the noise waves will not cause operation of the relay M because it has been readjusted by operation of A to be inoperative by noise. Auxiliary relay A which 1s more sensitive than master relay M may operate to close its normally opened contacts 77 causing a circuit extending from ground through battery 74, resistance 75, winding 71 of relay A and contact 77 of relay A to ground, to be set up. This, however, will have no effect on the relay A as that relay has already been operated to close its contacts 67 in response to the previous operation of relay A closing an energizing circuit for the winding 71 of relay A through contacts 76 of relay A The reduction in the sensitivity in the master echo suppressor relays M and M has been controlled, therefore b the greater noise on the path EA alone. I however, the noise on the transmission path WA had'been greater than the noise on the transmission path EA the auxiliary relay A would have been responsive thereto to cause the operation of relay A and the release of relay A to remove the normal back bias and thus to decrease the sensitivity of these relays in a manner similar to that described in connection with relay A WVhen the transmission of speech waves over path EA ceases, the wave-controlled, relay-'controlling'device 6 in control circuit 5 connected thereto, will'release its energization due to speech but retain that due to noise, partially deenergizing the operating windings 7 and 8 of master relay h 1 and auxiliary relay A respectively. Relay 1 1 will release but A will remain operatedby be again short-circuited. The contactsfil of relay M will simultaneously return to their normally opened condition thus breaking' the energizing circuit for holdover Winding 27 of relay H0 The condenser 49 which has been charged up during the energization of winding 27 by current from battery 42 will then start to discharge slowly through winding 27 and resistance 48 -maintaining the winding 27 energized for the additional holdover period defined above which is determined by the value of the condenser 49 and the value of the resistance 43.

Itelay HO will therefore remain operated for that additional period. Therefore, the short-circuiting connection 14 across the outgoing portion of the path WA will remainrclosed, and the energizing of the operating Winding 29 of the relay H0 byfalse operation of relay M by the continued open condition of the contacts 40 of relay H0 is prevented, for the same interval of time. At the end of this holdover interval of time, the relay H0 will release closing contacts 40 and opening contacts 18. The opening of contacts 18 of relay HO deenergizes the relay S causing the short-circuiting connection across the transmission path WA at the points 15 to be broken by the opening of the contacts 13 in the connection 1 1. The closing of the contacts 40 removes the break in the energizing circuit for the operating winding 29 of relay HO thus returning that relay to the condition where it may be operated under control of master echo suppressor relay M In the manner which has just been described, the combinations of relays shown in the drawings operate, when there is noise present ineither the transmission path EA or the transmission path WA, or both to reduce in one step the sensitivity of the relays of both master relays M and M to the point where they will just not be operated by impressed noise corresponding in magnitude to that on the transmission path on It is apparent, of course, that a closer ap- I proximation of continuous variation of the sensitivity of the master echo suppressor relays with noise could be obtained by using additional relays to cause the sensitivity to be changed in small steps. obtained, as shown in Fig. 2, by providing This may be additional steps in back bias on the master echo suppressor relays M and M controlled by a set of auxiliary relays A to A corresponding to auxiliary relays A to A in the circuit which has just been described, each set of relays having the required design to produce a different proportion of thereduction in the sensitivity of the master echo suppressor relays. In 2, the re lays A to A are identical with the relays A to AA, except that by suitable choice of the biasing elements, the relay A is made less sensitive than relay A and relay A is made less sensitive than relay A The elements of relays A to A corresponding to those of relays A to A; respectively are identified in Fig. 2 with the same reference characters identifying the elements of the latter relays but followed by a prime mark.

The operating winding 8' of auxiliary relay A is connected in series with the operating winding 8 of relay A and the operating winding 7 of the master relay M across the output of the amplifier-rectifier 6 in control circuit 5. Similarly, the operating winding 12 of auxiliary relay A is con nected in series with the operating winding 12 of auxiliary relay A and the operating winding 11 of master relay M across the output of the amplifier-rectifier in control circuit 9.

The biasing winding 70 of relay A, is energized inseries with the biasing winding 70 of relay A by current from battery 72 through resistance 73. The operating winding 71 of relay A is arranged to be ener gized by current from the battery 74 through resistance 75 when either the normally open contacts 76 of auxiliary relay A or the normally open contacts 77 of relay A are closed by operation of the respective relays.

The biasing winding 61 of auxiliary relay A in series with the biasing winding 61 of auxiliary relay A is energized by current from battery 63 through the resistance 64-. The operating winding 62 of relay A is normally energized by current from the battery 65 over apath extending from ground through battery 65, resistance 66', winding 62 and the normally closed contacts 69 of the auxiliary relay A to ground, so as to hold the relay contacts of relay i closed. Similarly, the operating winding 62 of relay A is normally energized by current from the battery over a path extending from ground through battery 65, resistance 66, winding 62 and the normally closed contacts 69 of the auxiliary relay A to ground, so as to hold the relay contacts 60 of relay A closed.

As in Fig. 1, the biasing winding 51 of master echo suppressor relay M is maintained energized at all times by current from the battery 52 through the resistance 52 so that the relay would operate in the least sensitive condition were it not for wind ing 56, and the biasing winding 54 of master relay M is maintained energized at all times by current from the battery 55 through the resistance 55 so that the master relay M would operate in the least sensitive condition, were it not for Winding 57. The third winding 56 of relay M so wound as to oppose the action of the operating winding 7 thereof, and the third winding 57 of relay M so wound as to oppose the action of the operating winding 11 thereof, are normally energized in series by current from battery 58 over a path extending from ground through battery 58, resistance 59, winding 57 of relay M winding 56 of relay M normally closed contacts 60 of relay A and normally closed contacts 60 of relay A to ground, thus applying back bias to these re lays opposing the normal bias produced by the biasing windings 51 and 54 of the respec tive relays. The values of the elements in this series circuit are so chosen that the amount of back bias normally applied is the same for each master relay, and suficient to allow the relays M and IV} to operate in their most sensitive condition.

The sensitivities of the auxiliary relays A I 2 are adjusted by proper choice of the elements in their circuits so that each relay will operate to close and maintain closed its contacts 76, 7 7 respectively whenever the amount of noise impressed on its operating Winding 8 or 11 from the output of the associated amplifier-rectifier device 6 or 10 exceeds a given low value. Also, the sensitivities of the auxiliary relay A A are respectively adjusted so that each relay will operate to close its contacts? 6 or 77 whenever the amount oi noise impressed upon its operating winding 8 or 11, exceeds a given higher value. Let it be supposed that noise is present on the path EA. in suiiicient amount so that when it is amplified and rectified by the amplifier-rectifier device 6 in control circuit 5, it will cause the operation of relay A but not the operation of relay A This will start the chain of operations, as described above in connection v with Fig. 1, culminating in the release of the relay A to open its normally closed contacts 60. The opening of contacts 60 will remove the normal short circuit around the resistance 90, thus connecting that resistance 90 in the energizing circuit for the third windings 56 and 57 of the master relays M and M respectively, the energizing circuit for these windings now extending from ground through battery 58, resistance 59, winding 57 of relay M winding 56 of relay M,,nor1nally closed contacts 60 of relay A and resistance 90 to ground. The value of the resistance 90 is so selected that the amount of back bias removed from the re- III lays M and M by its inclusion in the energizing circuit for their third windings is just suflicient to prevent operation of the latter relays by the noise Waves of said given lower amount impressed on their operating windings while allowing operation by the speech waves whenever speech overrides the noise.

Now let it be supposed that the amount of noise impressed from the output of the amplifier-rectifier 6 in control circuit 5 on the series circuit comprising the operating winding 7 of relay M the operating winding 8 of auxiliary relay A and operating winding 8 of auxiliary relay A increases to such value as will cause the operation of relay A in addition to the relay A so as to cause the contacts 76 of relay A to close. This will start the chain of operations culminating in the opening of normally closed contacts of auxilary relay A through the operation of relay A and the consequent release of relay A The opening of contacts 60 of auxiliary relay A will remove the normal short circuit around resistance 90 causing the resistance 90 to be connected in the energizing circuit for the third windings 56 and 57 of relays M and M respectively, in series with the resistance 90, the energizing circuit for these windings now extending from grounc through battery 58, resistance 59, winding 57 of relay M winding 56 of relay M resistance 90 and resistance 90 to ground. The value of the resistance 90' is so selected that its inclusion in the energizing circuit for the third windings of relays M and M in series with the resistance 90, will cause the amount of back bias removed from the relays M and M to be just sufiicient so that these relays will not be operated by said greater amount of noise impressed upon their operating windings. By using additional chains of relays corresponding to relays A to A, and additional resistances corresponding to resistances 90 and 90 and connectedin similar manner, it may be seen that the sensitivity of the master relays M and M may be changed in still smaller steps so as to obtain a closer approximation of continuous variation in sensitivity with the amount of noise.

The auxiliary relays A A, may be dispensed with in systems not subject to noises of the type for which those relays were provided, that is, noises which have pronounced low frequency components. If the relays A A are not used, the windings of the auxiliary relay A would be controlled directly from the contacts of the auxiliary relay A or A and the windings of the auxiliary relay A would be controlled directly from the contacts of the auxiliary relays A or A In the system of the invention which has been illustrated and described, an additional one-way amplifying device (2 or 4) is connected in each transmission path between the input of the associated control circuit (5 or 9) and the blocking points (21 or 15) in that path. These one-way devices besides providing additional amplification in the transmission paths, are utilized also for the purpose of preventing the short-circuiting of the outgoing portion of each path under control of the echo suppressor connected to the oppositely directed transmission path from affect ing the input to the echo suppressor associated with the first path. This can be accomplished in other ways, for example, by associating the output of the first amplifying device in each path (1 or with the input of the associated control circuit and the outgoing portion of the same path through a hybrid coil transformer and associated balancing network, or through a special bridge transformer network such as is disclosed in the copending application of B. G. Bjornson, Serial No. 451,520, filed July 29, 1930. Other modifications of the circuits which have been illustrated may be made by persons skilled in the art without departing from the spirit and cope of the invention as defined in the appended claims.

What is claimed is:

1. In a two-way signal transmission system, two one-way signal transmission paths for transmitting the signals in opposite directions in said system, means connected to each path and responsive to signal transmission therein effectively to disable the other path, and means for varying the sensitivity of both of said disabling means to operation in accordance with the amount of noise in the particular one of said paths in which the noise is a maximum so as to maintain at all times said disabling means in the most sensitive condition possible as regards operation by signals while preventing false operation by noise.

2. In a two-way signal transmission system, two one-way signal transmission paths subject to disturbing noise, means connected to each of said paths and responsive to signal transmission therein effectively to disable the other path, means for preventing false operation of either of the disabling means by the noise transmitted over said paths, and means responsive to the noise on the transmission path on which the noise is a maximum to vary the sensitivity of both disabling means.

3. In a system for suppressing echoes in a two-way signal transmission circuit comprising one-way paths for transmitting signals in opposite directions, a control device, connected to each of said one-way paths and responsive to waves transmitted thereover, relay means controlled by the device connected to each path and adapted to disable effectively the oppositely directed path, and

means responsive to the noise on the path only in which it is a maximum, to reduce the operating sensitivity of both of said relay means so as to prevent false operation by noise. 1

4. The system of claim 3 and in which the last-mentioned means comprises means for varying the sensitivity of both relay means simultaneously and by the same amount. 5. The system of claim 3 and in which the last-mentioned means is designed to be inoperative by speech waves.

6. The system of claim 3 and in which the last-mentioned means comprises a chain of relays operated in sequence in response to the noise received from said paths above a certain amplitude level, and having difiierent operating and holdover times determined by the type of noise against which protection is desired, to apply biasing potentials to wind.- ings of said relay means to reduce their sensitivity so as to prevent false operation by said noise.

7. The system of claim 3 in which lastinentioned means comprises a plurality of relays of different operating sensitivities, responsive to the noise received from 'said paths to provide successive steps of reduction of the sensitivity of said relaymeans as the amount of said noise increases to maintain said relay means at maximum sensitivity as regards operation by speech at all times while preventing operation by said noise.

8. The system'of claim 3 and in which the last-mentioned means comprises a chain of relays operated in sequence in response to noise waves received from said paths to bias said relay means so as to vary their sensitivity in accordance with the amplitude of said noise waves.

9. The system of claim 3 and in Which the last-mentioned means comprisesa chain of relays operated in sequence in response to noise waves received from said paths to bias said relay means so as to vary their-sensitivity in accordance with the amplitude of said noise waves, said relays having such operating and holdover times as to prevent operation of the last relay in the chain by said signals.

10. In combination, means connected to one signal transmission path and responsive to signals transmitted thereover forcontrolling the transmitting efficiency of another trans- 7 mission path, and means for controlling the sensitivity of said controlling means in accordance with the level of noise on said one path comprising a noise master relay 0011-.

thereover, means contrelled by operation of' said device to disable efiectively the oppositely one-way path, and means for pr venting at all times operation of said disabling means by noise while reducing its sensitivity to operation in accordance with the level of noise in said paths, said means comprising a noise master relay in the output of each relay-controlling device and adapted tooperate whenever the noise on the associated path rises above a certain level, a second relay adapted to be operated by operation or" either noise master relay, and having a. given holdof the g over in its time of operation, a third relay adapted to be operated if said second relay remains operated for a predetermined length of time, and means controlled by operation of said third relay to reduce the sensitivity of bothdisabling means to operation.

In witness whereof, we hereunto subscribe our names this 9th day of September,'1930.

BJijRN G. BJQRNSON. ARTHUR W. HORTQN, J R. ANDREW C. NORl/VINE.

nected to said one path and adapted to be operated whenever the noise level therein exceeds a certain level, a second relay adapted to be operated by operation or" said master relay and having a given holdover operation time, a third relay adapted to be operated when said second relay is maintained continuously operated for a predetermined time, and p 

